As the first quarter of 2016 comes to a close, the battle for control of the U.S. Senate is already underway. Candidates will have to file quarterly reports of their campaigns’ finances over the next two weeks. But we already have data on outside spending (that is, expenditures by anyone other than candidates and parties), since it must be reported every few days. While it’s still early in the election, the trends we predicted are already starting to appear. The money is coming from groups fueled by donations that are dozens or hundreds of times higher than federal candidate contribution limits, even when the group is dedicated solely to electing a single candidate. And many of the sources of the spending are hidden from the public.

In a harbinger of what is likely to be one of the nation’s most expensive Senate races, outside groups have spent $5.8 million in Ohio this year — more than double what was spent six years ago, and the most spending from outside groups in any congressional race so far in 2016. Freshman Sen. Rob Portman (R) is defending his seat, and former Gov. Ted Strickland recently won the Democratic primary race, defeating Cincinnati City Council Member P.G. Sittenfeld. The contest is widely considered to be one of the most competitive in the nation, as the parties struggle for control of the Senate.

The last time this seat was up for grabs was the year the Citizens United era began and super PACs were born. In 2010, Portman ran against Strickland’s lieutenant governor, Lee Fisher, for the open seat. For that entire year, nonparty outside spending totaled just $2.6 million — less than half what groups have already spent just through the end of March this year.

The great majority of the 2016 spending has come from conservative groups attacking Strickland, who has long been the presumptive Democratic nominee. The two biggest spenders so far are both part of the political network spearheaded by the billionaire industrialist Koch brothers: a super PAC called Freedom Partners Action Fund and the nonprofit Americans for Prosperity. The two groups have spent a combined $3.4 million hammering Strickland.

Spending by dark money groups which hide the identities of their donors is a significant factor in Ohio, accounting for 36 percent of outside spending. This has been driven by Americans for Prosperity, one of the highest-spending secret-money groups in the 2012 election, and by the U.S. Chamber of Commerce, which has been at or near the top of the list of highest-spending dark-money groups in every election since 2008.

But these reported spending totals are incomplete, because not all types of election spending are required to be reported to the FEC. In Ohio, the American Chemistry Council, an industry trade group, has runads supporting Portman. No one knows how much they have spent or who their donors are.

Single-candidate groups are also active in the state. Donors can give these “buddy groups” checks of any size, all to be spent on their favored candidate, providing a convenient way around the strict limits on the size of direct contributions to candidates. In Ohio, both Portman and Sittenfeld have benefited from buddy groups’ activities, whose spending in the state comprises around 14 percent of outside expenditures.

The group boosting Portman, Fighting for Ohio Fund, was founded by Barry Bennett, who worked for Portman’s campaigns in the past. Fighting for Ohio has attracted several six-figure donors, including major GOP donors from the financial industry and an Ohio dark-money group called Freedom Vote, Inc.

Sittenfeld’s long-shot campaign was supported by six-figure spending from a buddy group called New Leadership for Ohio. That super PAC’s major sources of funds include $100,000 from Sittenfeld’s father and $325,000 from the family of Sittenfeld’s campaign manager.

These six-figure donations are dwarfed by what’s possible in the plutocrat-dominated world of outside spending, however. For example, the biggest spender in Ohio, the conservative Freedom Partners Action Fund, took in $3 million from just one donor: Charles Koch. The pro-Democrat Senate Majority PAC, which has attacked Portman, has multiple $1 million donors. As Election Day comes closer, this race is only likely to attract more and more mega-donors.

The Brennan Center will continue to monitor election spending in Ohio and other competitive races this year. The trends we’re already seeing indicate further deterioration in two pillars of our campaign finance system, contribution limits and disclosure. As the sources of election spending become increasingly concentrated and hidden, the case for reform grows all the more compelling.